The present invention relates to compounds, having a low coefficient of thermal expansion, for coating electrical and electronic components (drop coating compounds).
Various electronic components, particularly integrated circuits, are typically constructed on a substrate, for example, a semiconductor material, in a process that includes many steps. After the electronic component is created, it is glued, soldered, alloyed and, then, provided with electrical terminals that include fine wires. To provide protection for the circuit and exposed wires, it is known to enclose the components in a housing. To further protect the components, or instead of a housing, the components can be cast out or coated with an electrically insulating plastic compound.
The components, however, can also be used as "bare", or unhoused ICs. But, when so used, the component must be protected against environmental influences, for example, moisture, dust, and mechanical damage, after they are integrated in a hybrid circuit, a device, or a transport means. One technique for protecting such components is through use of a drop coating. In the process or drop coating, thermoset resins are dripped onto the regions, or circuits, to be protected, and the resin is then cured.
Thermoset resins that are used as a drop coating must satisfy a number of requirements. In particular, the resin, or any compound for that matter, that is to be in direct contact with the component to be coated cannot adversely affect the component or cause the component to deteriorate. Furthermore, during the curing process, as well as during later operation, the compound, used for drop coating, cannot, among other things, exhibit an excessively large volume change due to temperature gradients. If the coefficient of thermal expansion and glass temperature, lying in the region of the operating temperatures, of the compound is too large this can result in, among other things, a tearing of the wires and cause the component to malfunction.
It has been found that epoxy resins satisfy the aforementioned requirements when used in combination with an inorganic filler. For example, it is known to use a drop coating compound that is filled with a two-component epoxy resin. However, a great disadvantage of such resins is the necessity to mix the resin with a hardening component before it is used. This is due to the fact that pre-mixed resins can only be worked for a limited time period because the curing process, that proceeds at ambient temperature, causes the viscosity of the compound to increase.
An attempt to overcome this problem is to use pre-mixed products that are sold in a frozen state, what are referred to as "frozen products". This thereby shifts the mixing from the user to the manufacturer. But, these thermoset resins, that must be thawed before their use, also exhibit several disadvantages. Such resins only have a limited time within which they can be processed and accordingly they provide only a short molding time for application to the devices. Furthermore, a thorough cleaning of reactive resin residues from all parts that come into contact with the resin is necessary in intervals that are just as short in order to preserve the functionability. Therefore, a continuous production process is thus made more difficult. Moreover, the pre-mixed frozen products exhibit other technical and economical disadvantages.